docs: update keto examples to use subjectsets

[DavudSafarli]

related to https://github.com/ory-corp/cloud/issues/10677

• Update Keto examples to use SubjectSet instead of SubjectID
• Use more common names in examples, like Alice and Bob
• Adds missing dark theme colors to prism for keto code blocks
• Change the colors for keto code blocks to match the theme we use in OPL editor
• Update the prism.js for keto language highlighting for small adjustments

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[zepatrik]

Should we rewrite all tuples to natural?

[DavudSafarli]

natural reads better for sure, especially with highlight.

Maybe we can have Tabs, where user can switch between Tuple / Natural?

[DavudSafarli]

How would you phrase this kind of relationships with natural language?

values:id#set_value@(values:id2#set_value)